Extract attachment IDs from selection, reinsert as gallery shortcode.

gallery-conversion.php

<?php
//Plugin Name: Convert to Gallery
//Description: Extract attachment IDs from selection, reinsert as gallery shortcode. Use "convert to gallery" button in Text mode. Usefull if you accidentally bulk insert a number of images instead of inserting as gallery.

add_action( 'admin_print_footer_scripts', 'convert2gallery_quicktag', 100 );
function convert2gallery_quicktag() {
	?>
	<script type="text/javascript">

		QTags.addButton( 'convert2gallery', 'convert to gallery', extract_and_convert );

			function extract_and_convert(e, canvas, ed) {

				var sel, startPos, endPos, scrollTop, text;

				if ( document.selection ) { //IE
					canvas.focus();
					sel = document.selection.createRange();

					selection = sel.text;
					content = extract_attachment_ids( selection );

					sel.text = content;
					canvas.focus();
				} else if ( canvas.selectionStart || canvas.selectionStart == '0' ) { // FF, WebKit, Opera
					text = canvas.value;
					startPos = canvas.selectionStart;
					endPos = canvas.selectionEnd;
					scrollTop = canvas.scrollTop;

					selection = text.substring( startPos, endPos );
					content = extract_attachment_ids( selection );

					canvas.value = text.substring(0, startPos) + content + text.substring(endPos, text.length);

					canvas.focus();
					canvas.selectionStart = startPos + content.length;
					canvas.selectionEnd = startPos + content.length;
					canvas.scrollTop = scrollTop;
				} else {
					canvas.value += content;
					canvas.focus();
				}
				return true;

 			} //end extract_and_convert

 		function extract_attachment_ids( string ) {

			test = /wp-image-(\d*)/g.test( string );
			if ( ! test ) {
				alert( 'No attachment IDs found in selection.');
				return string;
			}

			var matches = string.match(/wp-image-(\d*)/g);

			gal = '[gallery ids="';
			for ( var ind in matches ) {
				var attid = matches[ind].match( /(\d+)/g );
				gal += attid[0] +',';
			}
			gal += '"]';

 			return gal;
 		}

	</script>
	<?php
}